There is a need in the field of RF technology to provide a filtering device that is cheap, of high-performance and can withstand high RF power. Such filtering devices are, for example, used in order to prevent out of band emissions of a transmitter and/or a receiver system. State of the art filtering devices of high performance and withstanding high power are expensive and typically quite bulky. As long as only low-power signals need to be filtered, there is a large variety of low-power filtering devices available. There is also a need for assisting an existing (low-power) filter where improved performance is needed immediately adjacent (in frequency terms) to an existing bandpass filter, for example. This can be either because the existing filter does not have the required stop-band attenuation or because its roll-off is insufficient.
Therefore it is of interest to provide a filtering device which combines an accuracy of a low-power filtering device with filtering to filter RF signals at a high-power level. Known high-accuracy filtering devices are, for example, surface acoustic wave filtering devices (SAW). Unfortunately the SAW filtering devices are not capable of handling high-power levels, exceeding several watts. For example, an SAW based duplexer Epcos B7462 is adapted to handle 1 watt of transmit power within a pass band, but only 10 milliwatts of out-of-band power.
The prior art also teaches use of a circulator in order to transform filtering characteristics of a low-power filtering device into transformed filtering characteristics at a high RF power level.
U.S. Pat. No. 5,473,295 to LK-Products OY, Finland teaches a SAW filter being coupled to a receiver (Rx) branch of a duplex filter. The provision of the SAW filter increases the stop band attenuation of the duplex filter. The SAW filter is configured as a notch filter. The SAW filter improves the rejection of a band-pass filter in a mobile radio telephone.
Jiguo Wen, et al. disclose “Suppression of Reflection Coefficients of Surface Acoustic Wave Filters using Quadrature Hybrids”, published in Ultrasonics, IEEE transactions on Ferroelectrics and Frequency Control, Volume 53, issue 10, pages 1912-1917. The Wenoto paper describes the use of quadrature hybrids to improve the input and output matching characteristics of a pair of identical SAW filters. The Hashimoto paper uses two quadrature hybrids.